1. Field of the Invention
The present invention relates to rotary drive systems and specifically those rotary drive systems driven by an electric motor which require control refinement by a closed loop electronic feedback system. In particular, the present invention relates to the mechanical design of a rotary drive system having a closed loop electronic feedback controller which varies the torque output from the electric motor for the purpose of rotating a rotatable body in a controlled manner.
2. Background of the Invention
A controlled rotary drive system typically comprises a mechanical system and a closed loop electronic control system. Such a mechanical system at least includes an electric motor, a rotatable body to be rotated in a controlled manner, a feedback device, e.g. a tachometer or encoder, and at least one shaft connecting and transferring torque from the motor to the rotatable body. In general, the electric motor armature or rotor is mechanically coupled to a motor shaft, hereinafter referred to as the drive shaft, and a first mechanical coupling is used to couple the drive shaft either directly to the body to be rotated in a controlled manner or to a further mechanical coupling which can include a capstan, a belt and pulley, a lead screw or gear coupling connecting the first mechanical coupling and the body to be rotated.
The closed loop electronic control system at least includes, a command signal representative of some desired characteristic of the body to be rotated in a controlled manner, e.g. representing position, velocity or torque, a feedback signal proportional to some parameter being measured by the feedback device, a signal processor for comparing the command signal with the feedback signal, an error signal, generated by the signal processor and an amplifier to amplify the generated error signal.
It is well known in control theory, see e.g. G. Beirnson, PRINCIPLES OF FEEDBACK CONTROL VOLUME 1, Wiley-Interscience Publication, John Wiley & Sons, 1988 pages 10 through 15 and VOLUME 2, 1988 Chapter 10, that the complex ratio of the system feedback signal to the system error signal is defined as the "loop transfer function". Such a loop transfer function is representative of the combined electro-mechanical performance of the loop, and is defined as a phasor having a magnitude equal to the "loop gain" and a phase angle equal to the "loop phase". The loop transfer function provides a quantitative means of indicating the ability of the loop to control the parameters of the body to be controlled. It is also well known that the loop transfer function varies in magnitude and phase as a function of the frequency of the error signal.
In general, it is necessary for a feedback loop to have a loop gain of at least unity in order to have effective control action on the body to be controlled and a phase lag of less that 180.degree. to remain stable. Since loop gain necessarily decreases with increasing frequency, and loop phase necessarily increases with increasing frequency, there exists for every control loop a "gain crossover frequency" above which the loop gain is less than unity and a "phase crossover frequency" above which the loop phase lag is greater than 180.degree.. It therefor follows that the an error signal having a frequency below the gain crossover frequency is magnified while, an error signal having a frequency above the gain crossover frequency is attenuated. The "bandwidth" of a control loop can be defined as the range of frequencies from zero up to the gain crossover frequency. In general it is a goal in control systems to provide maximum gain at low frequencies while at the same time providing maximum loop bandwidth. In addition, it is also a design goal that the loop phase crossover frequency be outside the loop bandwidth. Loop bandwidth is a means of quantifying how quickly the loop can vary the parameters being controlled.
A fundamental problem with the use of feedback control is that feedback can produce oscillations in either the control loop or in the mechanical system under control. Such oscillations occur when the frequency of the feedback signal is equal to or nearly equal to a system resonance frequency, i.e. a frequency where system gain is very large. A particular problem of controlled rotating systems is the excitation of mechanical torsion resonance frequencies of the drive mechanics and especially when these mechanical torsional resonance frequencies fall within the bandwidth of the control loop.
Mechanical oscillations occur as a result of different regions of the mechanical system having different angular positions and phases relative to each other such that some regions actually have opposing velocity vectors. This results in opposing torsional loads winding up the mechanical drive like a spring. The condition can be further reinforced by the control loop which also begins to oscillate at the mechanical torsional resonance frequency in an attempt to regain control. Mechanical resonance conditions can damage the mechanical system and at least result in a breakdown of effective motion control.
In general it is common in motion control systems to overcome mechanical oscillations electrically by the use of a notch filter to attenuate the system response at certain mechanical torsional resonance frequencies. A notch filter for use in motion control systems is described e.g. by B. Kuo and J. Tal, in, DC MOTORS AND CONTROL SYSTEMS, SRL Publishing Illinois, 1978 page 125, however such a filter can shift the loop phase crossover frequency inside the loop bandwidth adding electrical oscillation modes which must be otherwise compensated for.
In U.S. Pat. No. 4,507,592, R. Anderson teaches the use of stored motion profiles, stored in read-only memory and selected on the basis of input command signals and used to avoid excitation of certain mechanical resonance frequencies. This method requires prior knowledge of the velocity profiles required and may not account for load variations at the rotating body under control.
In U.S. Pat. No. 4,873,887, A. Rainer et al. teach the use of a torsion-vibration damper which alters the amplitude of mechanical oscillations by reducing system gain at certain torsional resonance frequencies but does not change the frequency of mechanical oscillation.
The parameters governing torsional resonance in a rotary drive system can be very complex, however for a simple shaft, see e.g. DC MOTORS AND CONTROL SYSTEMS by B. Kuo and J. Tal, SRL Publishing Illinois, page 124, it's fundamental torsional resonance frequency is determined by the shaft stiffness and its equivalent moment of inertia. According to Kuo and Tal, either an increase in shaft stiffness or a decrease in equivalent moment of inertia can each have the effect of increasing the fundamental torsional resonance frequency of the shaft. Furthermore, shaft stiffness and equivalent moment of inertia are completely determined by the shafts diameter, shear modulus of elasticity and length. In addition, the shaft diameter plays the most influential role on both parameters. It therefor follows that the fundamental torsional resonance frequency of a rotary drive can be increased by increasing the drive shaft diameter and shortening its length.
Given the fundamentals of controlled rotary drive systems outlined above and given the general goal of increasing control system bandwidth without exciting mechanical resonance frequencies, it is accordingly a general object of the present invention to increase the torsional resonance frequency of a mechanical rotational drive system in order that the closed loop electronic control system bandwidth can be increased.
It is a further object of the present invention to reduce the number of components of a mechanical rotational drive system thereby reducing system cost.
It is a still further object of the present invention to improve the accuracy of the motion feedback system by increasing the rotational drive stiffness such that the feedback device mounted to the drive has an increased response bandwidth.